1. Technical Field
This invention relates to an improved powder metallurgy composition, and specifically for an improved powder metallurgy composition suitable for use in sintering processes adapted to manufacture articles for the automotive industry. The invention hereafter described has particular relevance to the manufacture of valve seats, turbocharger bushings, and the like, but of course the invention should not be considered as being limited by the ultimate article into which the composition described herein is ultimately formed by sintering.
2. Related Art
In its simplest form, powder metallurgy is the science of mixing different quantities of powdered elemental metals, alloys, or metals or alloys having been subjected to diffusion bonding so that on sintering such mixtures, articles having desired wear resistance characteristics and stability at the elevated operating temperatures to which the ultimately formed components are often subjected can be cost effectively manufactured.
Powder metallurgy is, in general, is the process of compressing a predetermined powder metallurgical mixture under very great loads to create a what is known as a green compact, and then heating the green compact to a high temperature, often, but not necessarily, between the lowest melting point of any constituent in the mixture and the highest melting point, so as to cause some melting, or movement in terms of diffusion or infiltration, of at least one constituent in the mixture. On cooling (and it is to be mentioned that the heating and cooling stages may be very rapid or quite gradual, depending on the desired physical characteristics of the ultimate product), any residual molten or more fluid constituent solidifies.
It is to be mentioned at this stage that although the following description relates typically to sintering in a protective gas atmosphere or vacuum sintering, the invention has wider application, and indeed it is contemplated by the applicant that the invention could be equally applicable in other manufacturing techniques, such as powder forging, high velocity compaction, and the like.
One of the fundamental aspects of sintering, and in particular the powder metallurgical mixtures used to form sintered articles intended for high wear applications, is the relationship between what is known as the matrix and any hard phase that is incorporated to confer enhanced wear resistance. This relationship is likely to be atomic, structural, mechanical, and chemical, and therefore is fundamentally important in ultimately determining how the finished sintered article will behave in aggressive environments.
The matrix is essentially that substance or composition which effectively binds the overall composition together in the sintered article, said hard phase being dispersed randomly throughout the matrix to provide it with wear resistance characteristics. Accordingly, the matrix material is usually significantly softer than the hard phase, and usually (although not necessarily, depending on application), the concentration by weight of the matrix in the powder mixture, pre-compression, will usually be greater than the corresponding concentration by weight of the hard phase.
It is important to note here that volumetric percentages are sometimes used to express concentrations of constituents in powder mixtures, but these can be very different from the corresponding concentrations by weight, as the densities of the constituent metals or alloys can be significant, particularly as regards the hard phase.
In the remainder of this specification, weight percentage (wt %) is to be assumed unless specifically mentioned otherwise.
In general, the wt % of the hard phase is determined to a large extent by the type of article which is to be made. Valve seat inserts (VSI) typically demand a hard phase concentration of between 25-40 wt % due to the aggressive conditions in the immediate vicinity of internal combustion engine cylinders, whereas turbocharger and other bushings do not have such a high requirement for wear resistance, and accordingly a hard phase of between 8-18% is more common for these applications.
The present invention is to be considered as covering both such applications.
There is much prior art in this particular technological field, and some of the more relevant documents are discussed below.
EP-A-0 418 943, of common ownership herewith, describes sintered steel materials sintered from compacted mixtures comprising a hot working tool steel powder, iron powder and carbon additions in the form of graphite. The hot working tool steel is generally based upon one or more of those known as AISI H11, H12 and H13. Specifically, this patent covers a sintered ferrous material having a wt % composition as follows:
C0.7-1.3Si0.3-1.3Cr1.9-5.3Mo0.5-1.8V0.1-1.5Mn≦0.6Fethe remainder, apart fromincidental impurities.
EP-A-0 312 161, also of common ownership herewith, describes sintered steels made from compacted and sintered mixtures of high-speed tool steels forming the majority of the hard phase, iron powder and carbon additions in the form of graphite forming the majority of the matrix. The high-speed tool steels contemplated for use are generally based on the M3/2 class well known in the art. The sintered steels described in EP-A-0 312 161 are generally of lower carbon content than those described in EP-A-0 418 943. This is due to the fact that the alloying addition levels of the principal carbide forming elements of Mo, V and W are greater in the EP0312161 materials and this maintains the required high degree of wear resistance in applications such as valve seat inserts for example. As a result of the lower carbon level, there is also less of a problem in removing austenite from the structure after sintering. However, the problem with the alloys described in EP-A-0 312 161 is one of material cost due to the relatively high level of alloying additions. EP0312161 thus protects a sintered ferrous-based material having a matrix comprising a pressed and sintered powder, the powder having been pressed to greater than 80% of theoretical density from a mixture including two different ferrous-based powders, the mixture comprising between 40 and 70 wt % of a pre-alloyed powder having a composition in wt %
C0.45-1.05W2.7-6.2Mo2.8-6.2V2.8-3.2Cr3.8-4.5
Others 3 max, with Fe balance,
with between 60 and 30 wt % of an iron powder, optionally up to 5 wt % of one or more metallic sulphides, optionally up to 1 wt % of sulphur and carbon powder, such that the total carbon content of the sintered material lies in the range from 0.8 to 1.5 wt %.
As can be seen from the above, the concept of including a high speed tool steel in powder metallurgical compositions is well known.
The above provide examples of situations where very specific compositions are required to achieve a particular purpose or result in a particular sintered article with predetermined wear characteristics.